misc_helpers: fix zero_normalmem() for BL2_AT_EL3

The assertion in zero_normalmem() fails for BL2_AT_EL3.  This mode is
executed in EL3, so it should check sctlr_el3 instead of sctlr_el1.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>